1. Field of Invention
The present invention relates to the communication of digitally encoded data and in particular to a method of early starting the operation of a descrambler in which the data is scrambled in accordance with a binary sequence before transmission.
2. Description of Related Arts
In digital signal transmission pulse patterns can occur which have a disturbing DC component or whose energy component is particularly high at specific discrete frequencies. So as to avoid these pulse patterns, the digital signal which is to be transmitted is scrambled in a transmitter with a pseudo-random binary sequence (PRBS). In a receiver, descrambling is effected with the pseudo-random binary sequence (PRBS) which has previously been used in the transmitter. In other words, a scrambler in the transmitter and a descrambler in the receiver use the same pseudo-random binary sequence (PRBS).
FIG. 1 shows a typical scrambler/descrambler schematic diagram. Loading of an initialization sequence such as “100101010000000” into a pseudo-random binary sequence (PRBS) register 111, as indicated in FIG. 1, shall be initiated at the start of every predetermined transport packets such as eight transport packets.
In recent years, MPEG (Motion Picture Coding Experts Group) 2 standard has been widely employed to perform compression of video data such as in a broadcast system utilizing multi-channel digital signals and DVD. Two kinds of streams, i.e., a Transport Stream (TS) and a Program Stream (PS), are defined as a signal carrying a plurality of multiplexed contents such as video data and audio data. Particularly in digital broadcasting is used the transport stream. The transport stream is comprised of sequential packets each referred to as a transport packet.
FIG. 2A illustrates a format of a MPEG (Motion Picture Coding Experts Group) 2 transport packet. The total packet length of the MPEG2 transport packet is 188 bytes. This includes one synchronous byte (i.e. 47HEX). To provide an initialization signal for the descrambler, the MPEG2 synchronous byte of the first transport packet in a group of predetermined packets such as eight packets shall be bitwise inverted from 47HEX to B8HEX. In other words, synchronization is established by inverting once the synchronous byte in every predetermined packets such as eight packets using the synchronous byte (=47HEX) in the packet. To each MPEG2 transport packet (MPEG2 TS packet), an error correcting code of the Reed Solomon Code (204, 188) is added. Please refer to FIG. 2B which shows eight MPEG2 transport packets. The period of the PRBS shall therefore be 1632 bytes.
It has previously been proposed to scramble encoded video data prior to transmission by combining at an XOR gate the outputs of a coder and a PRBS (pseudo-random binary sequence) generator. The scrambled data can then be read only at a receiver having a matching descrambler. In order to descramble the data correctly it is necessary that the PRBS generator 110 in the receiver should be aligned with that in the transmitter so that corresponding bits in the pseudo-random sequence are applied to the same elements of data in the scrambler and descrambler. As described in the present applicant's earlier British applications numbers 8804552 and 8721603, this is achieved by searching for a video line synchronisation or frame alignment word, or other predetermined fixed component of the received data and aligning the PRBS generator accordingly. In other words, in order to let the data scrambling and data descrambling to work correctly, the operation of the scrambler and the descrambler have to be synchronized.
Assuming that the period of the PRBS is T such as eight transport packets, the pseudo-random binary sequence (PRBS) register of the scrambler will be reloaded again with a fixed initialization sequence such as “100101010000000” every eight transport packets. To provide an initialization signal for the scrambler, the MPEG2 synchronous byte of the first transport packet in a group of eight packets shall be bitwise inverted from 47HEX to B8HEX. In other words, the scrambler will be initialized with the fixed initialization sequence such as “100101010000000” every time at the start of B8HEX. Similarly, the descrambler will load the pseudo-random binary sequence (PRBS) register thereof with the fixed initialization sequence such as “100101010000000” and start its operation by searching for B8HEX. Please refer to FIG. 3A. If the receiver starts to toss out the valid data at time t0, the descrambler will not start until time t1. In other words, the worst-case latency time is T as indicated in FIG. 3B.
Accordingly, it would be advantageous to have an improved method of early starting the operation of a descrambler in which the data is scrambled in accordance with a binary sequence before transmission.